The present invention relates to an apparatus containing a cylinder, a piston displaceable in the cylinder, and a measuring transducer arranged essentially within the cylinder.
In its more specific aspects, the invention relates to an apparatus comprising a cylinder, a piston displaceably guided in such cylinder, the piston being positionally shiftable by a suitable flow medium and having an elongate or longitudinal opening. Further, there is provided a measuring transducer arranged essentially within the cylinder and having two parts which are displaceable towards one another, one such part being attached at the cylinder and protruding into the longitudinal opening of the piston, and the other such part being secured at the piston or being constituted by the piston itself.
Such cylinder arrangements are used, by way of example, for lifting loads, for shutting off closures or for positioning tools of building and machine tools.
Swiss Pat. No. 488,999 and the corresponding U.S. Pat. No. 3,654,549, disclose an apparatus containing a cylinder and a measuring transducer installed within such cylinder. With one embodiment of the state-of-the-art equipment there is used an inductive measuring transducer having one part secured at the cylinder and the other part at the piston. The part of the measuring transducer which is secured at the cylinder comprises two hollow tandemly arranged coils disposed essentially coaxially with respect to the cylinder. The measuring transducer part attached at the piston comprises a ferromagnetic core, which, depending upon the position of the piston, penetrates to a greater or lesser extent into both coils. Both of these coils are connected with an alternating-current source and a bridge circuit, from which there can be tapped-off a signal constituting a measure for the piston position.
The coils required for the heretofore known equipment are relatively expensive. This is particularly then the case if the piston has a large displacement stroke. A further drawback of the prior art equipment resides in the fact that, at least three coil connections must be led out of the cylinder by means of electrically insulating bushings or equivalent structure. These bushings are particularly then expensive and prone to disturbance if large pressures are used in the cylinder. It is furthermore to be mentioned that an apparatus working with inductive measuring transducers can be sensitive to external magnetic fields and ferromagnetic parts of other machines, so that measurement errors can easily arise.